[0001] This invention relates to optical fibre cables, particularly but not exclusively
such cables suitable for long distance underwater transmission systems.
[0002] We currently manufacture a cable similar to the one described and claimed in British
Patent Application No.8229562 (C.S. Parfree et al 20)(Serial No.2l28358). The optical
fibre package comprises a plurality of secondary coated optical fibres e.g. coated
with Nylon l2 (RTM) over a primary Sylguard (RTM) coating which are helically laid
up around a kingwire and held in a package by a whipping. This has proved satisfactory,
but with the advent of acrylate coated fibres i.e. fibres with one or more layers
of acrylic coating over the bare fibre, which are now in widescale production, attempts
to substitute these fibres for the Nylon coated fibres failed because the transmission
loss was unacceptably high. This was due to the lack of buffering between the acrylate
fibres and the rest of the package.
[0003] One arrangement for a cable element for use in a submarine cable is proposed in our
co-pending patent application (as yet unpublished at the time of making the present
application) S.R. Barnes et al 4-3-2 (British Patent Application No.85l6290) in which
acrylate coated fibres are drawn through a forming die together with a central kingwire,
and a thermoplastic polymer, particularly one marketed under the trade name Hytrel,
is extruded around and between the fibres and kingwire in a "one-shot" operation.
This encapsulates the fibres and kingwire in a straight lay configuration in one pass
without any subsequent extrusion process and the element so formed can be used directly
in the longitudinal cavity of a submarine cable such as the one described in Parfree
20, in place of the optical fibre package there described.
[0004] This technique is difficult to control in such a way that the original low loss of
the fibres is guaranteed in the finished element and stresses are imparted to the
fibres as they pass through the extrusion head which are difficult to control or quantify.
[0005] Whilst this has proved suitable for relatively short lengths e.g. up to 5Km, the
losses and the yield are still not as good as we have obtained with the previous package
referred to above.
[0006] An alternative approach is disclosed in our published British Patent 2ll3903B (L.R.
Spicer 26) in which optical fibres and a coated string are pulled through precision
tooling having a bore for each fibre, whereby the optical fibres are partially embedded
in the outer periphery of low-density-polyethelene-coated string which has been heat
softened, and extruding an insulating sheath over the partially embedded fibres, which
sheath can for example be high density polyethelene.
[0007] None of the proposals discussed above enables long lengths e.g. 50km of cable to
be produced in a continuous process. For example using the above processes we achieved
only 5kms before a fibre broke due to dust build up in the tooling. Also spliced fibres
have a bulge in the coating so they would not pass through the precision tooling of
e.g. 2ll3903B.
[0008] Another proposal is set forth in published British Patent Specification 2l36350A
in which a central strength member is heated and a first layer of thermoplastic elastomer
is extruded onto the heated central strength member. Optical fibres are laid along
a helical path onto the first layer with a planetary motion. A second layer of thermoplastic
elastomer is extruded over the fibres and merges with the first layer, and then a
protective nylon sheath is extruded around the elastomer. For undersea applications
the central strength member may be a central conductor of a coaxial cable, for low
frequency signalling of information for surveillance, maintenance and control.
[0009] However the equipment is complex requiring as it does a rotating die and rotating
bobbins and lay plate equipment (Figs. 3 and 4) requiring a completely enclosed environment
for each fibre during passage from the bobbin to the die. Furthermore each fibre passes
through four separate guides during its passage from the supply bobbin to the closing
die.
[0010] It is an object of the present invention to overcome the above problems in a simple
effective manner to provide a cable element of very long length e.g. 50kms and above,
suitable for a submarine cable and using acrylate or equivalent coated fibres.
[0011] According to the present invention there is provided an optical fibre cable including
a tubular tensile strength member defining a longitudinal chamber and, within said
chamber, an optical fibre cable element comprising: a plurality of optical conductors
each having over the bare conductor a first protective coating which is relatively
thin characterised by a central string wire having a buffer coating into which the
fibres have been embedded so that they are spaced from the central string and from
each other with a zero angle of lay, and a second buffer coating enveloping the embedded
fibres, said cable having a longitudinal water blocking material in the gap between
the element and the longitudinal chamber.
[0012] According to another aspect of the present invention there is provided a method of
making an optical fibre element in long lengths e.g. 50km characterised by comprising
providing a string member comprising a buffer material, providing a plurality of optical
fibres each having a protective coating thereon, drawing the string member and the
fibres through a fixed lay plate and a tapered-aperture die spaced from the lay plate,
in that order, the lay plate serving to maintain the fibres in a predetermined-spaced-apart
relationship as they enter the die, causing said fibres to become at least partially
embedded in the buffer material, and extruding a buffer envelope over the central
string member and embedded fibres to encapsulate the fibres in the buffer envelope
and buffer coating.
[0013] According to a further aspect of the present invention there is provided a method
of making an optical fibre cable comprising providing or making an element as claimed
in claim l, or claim 7 respectively, inserting the element into the open channel of
elongate metal member, closing the metal member around the element with a small gap,
said gap being water blocked with a water blocking medium, applying strength member
wires around the closed metal member, and extruding a plastics jacket over the wires.
[0014] In order that the invention can be clearly understood reference will now be made
to the accompanying drawings, in which:-
Fig. l shows apparatus for carrying out the method of making the cable according to
an embodiment of the present invention;
Fig. 2 shows in cross section a submarine telecommunications cable element made by
the apparatus and method shown in Fig. l, and
Fig. 3 shows in cross section a submarine cable embodying the element of Fig. 2 and
in accordance with an embodiment of the invention, and
Fig. 4 shows a method of making the cable of Fig. 3.
[0015] Referring to Fig. l of the drawings the apparatus for making the cable element shown
in Fig. 2 comprises a bobbin l carrying a kingwire 2 made of steel, Nylon (RTM) or
Kevlar (RTM) or other high tensile material and coated with a buffer coating 3 of
a thermoplastic elastomer, which is soft in comparison with the acrylate coating 4
on optical fibres 5. We prefer to use an elastomer sold under the trade name Hytrel.
The acrylate coated fibres 5 are carried on bobbins such as 6.
[0016] The coated kingwire l and the coated fibres 5 are guided into and drawn through a
die 7 which partially embeds the fibres 5 into the periphery 8 of the buffer coating
3. The coated kingwire is also passed through a heater tube 9 which heats the coating
on the kingwire to a temperature of about l60°C ± l0°C which softens the coating to
a point at which the fibres become partially embedded into the coating 3 by the die.
[0017] The die is shaped as shown in the Fig. lA and has an internal diameter Aʹ which is,
at its narrowest point just short of the face 7b, 300um larger than the diameter A
of the coated kingwire 2. Fig. lA shows a cross section of the die 7 with the fibres
5 visible and being partially embedded in the periphery of the coated kingwire 2.
[0018] The fibres 5 are guided into the die 7 by a fixed lay plate l0 which has a number
(twelve) of guide slots such as ll each several millimetres wide (holes could be used
instead) in which the fibres are loosely guided in a predetermined angularly-spaced-apart
relationship around the kingwire 2 as they all enter the die 7 and this equal spacing
is maintained within the die 7 by the tension in the fibres and the fixed lay plate.
This tension lies in the range l0 to l00 grams and the back-tension in the kingwire
is carefully controlled, together with the temperature of the heater 9 to ensure that
the net longitudinal compressive or tensile strain on the fibres in the straight,
finished element is minimal and does not exceed 0.05%.
[0019] The radius of curvature of the surface 7a and of the walls of grooves or holes ll
are such that the curvature imposed upon the fibre restricts the strain on the outside
surface of the fibre to significantly less than the proof strain. In the present embodiment
the radius is about l0mm which gives ample margin for a l.00% proof-tested fibre requiring
a minimum diameter of 6.2mm.
[0020] Following the die 7 is a crosshead extruder l4 of completely conventional design
and this extrudes a second buffer coating l2 of a soft thermoplastic polymer over
the partially embedded fibres 5. The polymer is preferably the same material as was
used for the first buffer coating 3, namely one sold under the trade name Hytrel.
[0021] The shore A hardness of the buffer coatings preferably lies in the range 70 to 90
at ambient temperature.
[0022] In the preferred embodiment described the coated fibres 5 have a diameter of 280
to 300um, the diameter A of the coated kingwire lies in the range l.4 to l.8mm, the
diameter C of the second buffer coating lies in the range 3.l to 3.4mm and the diameter
B of the kingwire has in the range 0.6 to l.0mm, so both coatings 3 and l2 are many
times thicker than the coated fibre diameter.
[0023] Fig. 3 shows in cross section a deep water submarine cable incorporating the element
of Fig. 2. The element is shown schematically and represented by the reference numeral
20. The element is housed in a longitudinal cavity 2l defined by a tubular metal member
22 with a small space 23 which is water blocked either continuously or intermittently
along the length of the cable with a viscous water blocking material 23A such as that
sold under the trade name HYVIS 2000. The metal member 22 can be of copper or aluminium
and can be made by closing an extruded "C" section element. The cavity 2l has a diameter
of about 0.2 to 0.6mm larger than that of the element 20, preferably 0.3mm larger.
[0024] Around the member 22 are two layers 24 and 25 of tensile strength wires, the inner
layer 24 having a few relatively thick wire and the layer 25 having over twice as
many relatively thinner wires.
[0025] Around the strength member wire is extruded a dielectric material 26 such as polyethelene
to insulate the metal member 22 from the water so that it can be used to carry electricity
at about 7000 Volts from end to end of the system to power regenerators of the system.
[0026] In manufacturing the cable of Fig. 3, the processor of Fig. 4 is used. The element
20 is drawn from a reel 20A through a water blocking station 29 which smears the surface
of the element 20 with a thin, e.g. 0.2 to 0.4mm thick, layer of the water blocking
medium 23A. The smeared element 20 is then guided into the open channel of the metal
member 22, which has a "C" shaped cross section, in a station 30. Closing station
3l then closes the "C" section member 22 and the whole passes through a forming die
3lA which slightly reduces the closed metal member to a desired outer diameter and
ensures the smear of water blocking medium 23A fills the gap 23.
[0027] Then the cable passes through a stranding machine having a first part 32 which applies
the first layer of strength member wires 24 from reels 24A and a second part 33 which
applies the second layer of strength member wires 25 from reels 25A. In between stations
52A and 33A wipe silicone rubber onto the first and second layers of wires to water
block them. The cable then passes through an extruder 34 to extrude the outer jacket
26 over the wires.
[0028] We have found that a submarine cable as described has a low loss, e.g. better than
0.4dB per Kilometer over a temperature range of 4°C to 20°C, and at a wavelength of
l3l0nm..
[0029] The construction limits the minimum bend radius to 0.6 metres and this enables the
very low loss of the package of Fig. 2 to be maintained in the finished cable.
[0030] We have also devised a simple stripping and splicing method. We have discovered that
for a material like Hytrel it is possible to heat the end of the element 20 and the
buffer coatings appear to creep away from the coated fibres thus exposing them for
subsequent splicing using conventional techniques. In particular the end portion of
element 20 can be heated on a hot plate to about 200°C and this effectively removes
the buffer coatings prior to splicing.
1. An optical fibre cable including a tubular tensile strength member defining a longitudinal
chamber and, within said chamber, an optical fibre cable element comprising: a plurality
of optical conductors each having over the bare conductor a first protective coating
which is relatively thin characterised by a central string wire having a buffer coating
into which the fibres have been embedded so that they are spaced from the central
string and from each other with a zero angle of lay, and a second buffer coating enveloping
the embedded fibres, said cable having a longitudinal water blocking material in the
gap between the element and the longitudinal chamber.
2. A method of making an optical fibre element in long lengths e.g. 50km characterised
by comprising providing a string member comprising a buffer material, providing a
plurality of optical fibres each having a protective coating thereon, drawing the
string member and the fibres through a fixed lay plate and a tapered-aperture die
spaced from the lay plate, in that order, the lay plate serving to maintain the fibres
in a predetermined-spaced-apart relationship as they enter the die, causing said fibres
to become at least partially embedded in the buffer material, and extruding a buffer
envelope over the central string member and embedded fibres to encapsulate the fibres
in the buffer envelope and buffer coating.
3. A method as claimed in claim 2, characterised by said protective coating is an
acrylate material.
4. A method as claimed in claim 2, characterised in that the protective coating comprises
an inner layer of relatively soft material and an outer layer of a relatively hard
material.
5. A method as claimed in claim 2 characterised in that the buffer coating comprises
a thermoplastic elastomer, the method including the step of heating and softening
the elastomer to facilitate embedding the fibres therein.
6. An optical fibre cable comprising a tubular tensile strength member defining a
longitudinal chamber, and characterised in that there is within said chamber, an optical
fibre element made by a method as claimed in claim 2.
7. A method of making an optical fibre cable comprising providing or making an element
as claimed in claim l, or claim 7 respectively, characterised by inserting the element
into the open channel of elongate metal member, closing the metal member around the
element with a small gap, said gap being water blocked with a water blocking medium,
applying strength member wires around the closed metal member, and extruding a plastics
jacket over the wires.